The trend in building design today is to structures of ever increasing energy efficiency. In view of the continuing increase in energy costs, architects, builders and buyers alike are eager to find ways of reducing the cost of heating their homes and office buildings in winter and cooling them in summer.
The principal approach taken has been to make the construction of, for example, a house, more tight fitting and to provide for greater use of insulation. By reducing the gaps and spaces that arise at such places as foundations, walls, windows, doors, etc., infiltration of outside air can be reduced. Additionally, by increasing the amount and quality of the insulation between walls, in ceilings, etc., heat transfer out of and into the house can be limited. The result is that less heat is lost in winter and accumulated in summer, and accordingly, the cost for maintaining the house at a comfortable temperature is minimized.
However, by making the house "airtight", a potential hazard is created. Living space generates and accumulates pollutants which if not eliminated, would, at best, be annoying, and, at worst, create a health hazard. For example, carbon monoxide and carbon dioxide are generated by the breathing of house occupants, and operation of gas and wood stoves and fire places. Chemical vapors such as formaldehyde are given off by building and furniture materials like plywood, adhesives, insulation and furniture padding. Further, chemical sprays such as pesticides and cleaning agents give off noxious fumes. Still further, water vapor and odors are produced by cooking, showering, bathroom use, and clothes laundering. Additionally, radioactive gases such as Radon, which arise from natural radioactive disintegration in soil and building brick and masonry, give rise to insidious carcinogenic pollution. If left unchecked, these pollutants could rise to dangerous levels.
To avoid this health threat, while at the some time minimizing energy costs, architects and builders have resorted to use of special ventilators. These ventilators don't simply introduce fresh air from outside the house, for to do so would defeat the energy saving strategy of tighter construction and increased insulation use. Rather, these ventilators include heat exchangers which conserve at least some of the energy expended in maintaining the house at a comfortable temperature. In operation, the ventilators gather a stream of fresh air from outside the house, but, before introducing it, preheat or precool it, depending on the season, with the stale air being exhausted. Most typically, the gathered fresh air is passed in heat exchange relation to the stale air so that heat may be transferred from the warmer to the cooler. This enables a portion of the energy expended in maintaining the room at a comfortable temperature to be conserved by either extracting heat from or adding heat to the stale air, before it is exhausted, and, respectively, either adding heat to or extracting heat from the fresh air being supplied.
William A. Shurcliff, a noted expert in the field of thermal efficiency in building design, has written on this subject. In his book entitled: AIR-TO-HEAT EXCHANGERS FOR HOMES, published by Brick Publishing Co., Inc., Andover, Mass., he describes in detail the operation and design of heat exchanging ventilators, and additionally, describes the construction and operation of a number of currently available types.
As pointed out by Shurcliff, the savings in energy costs that a heat exchanging ventilator can provide, as for example in time of winter, is the product of the ventilator's flow rate and the exchange efficiency, multiplied by, a constant that depends on the cost of energy to heat and the expected "degree-day" factor; i.e., need for heat, of the geographical area where the ventilator is to be used.
Shurcliff defines the flow rate of the ventilator as the volume of fresh air supplied by the ventilator per unit time. Further, he defines ventilator efficiency as the heat per unit weight of air transferred to the fresh, incoming air, divided by, the difference in heat per unit weight of air being exhausted and fresh air outside the house. In the absence of condensation in the stale air exhaust stream, this reduces to the difference in temperature between the fresh air introduced into the ventilated room and the outside air temperature, divided by, the difference in temperature between the stale air in the room and the fresh air outside.
Therefore, the greater the flow of air the ventilator can handle, and, in the case of winter heating, the higher the temperature of the fresh air supplied, the greater the dollar savings the ventilator can provide.
As also noted by Shurcliff, in order to determine whether a particular ventilator design will be cost effective, the dollar savings it produces must be compared to the ventilator's purchase price. According, not only must the operation of the ventilator be effective, i.e. high flow rate and high efficiency, but also, its purchase price, and therefore it cost of construction must be low if one is to maximize the return on investment.
As a further consideration, the ventilator should also have a form factor that renders its physical integration into the structure to be ventilated convenient. While, as shown by Shurcliff, a number of heat exchanging ventilators exist which provide acceptable flow rates and efficiencies, most are not of a size that permits convenient and unobtrusive installation; e.g., within the wall of a house or other building to be ventilated.